This form enables you to effect an express surrender of tenancy.
'Surrender' is the legal term for an agreement between the landlord and tenant that the tenancy will end.
A surrender can occur both during a periodic and fixed term tenancy. The basic idea is that landlords and tenants are free to end the tenancy if they both want to.
Ending a tenancy by mutual agreement is best done in writing if possible.* For the landlord a written agreement will prevent a dispute arising subsequently about whether there was an effective agreement to end the tenancy.
If it's a joint tenancy both tenants will need to sign the form.
*A tenancy may end by surrender even if the termination is not effected by deed. However this requires unequivocal conduct by both the landlord and tenant demonstrating acceptance that the tenancy has ended. You should seek legal advice if you need to know whether a surrender can be implied in light of the landlord's and tenant's actions.